1. Technical Field
The present invention relates to the field of microwave and RF electronics, and more particularly to broadband hybrid structures.
2. Related Art
A 180° hybrid is a component that provides a phase-shifted output of unbalanced RF signals. The 180° hybrid is an essential component for a multi-port vector network analyzer (VNA) that offers true differential measurement capability. Differential measurements are becoming more important due to greater use of differential components and circuits in the modern communications industry.
In order to provide a phase-shifted output, an unbalanced signal must be converted into two balanced signals that are later converted into two unbalanced output signals with equal amplitude and 180° phase shift. To create two balanced signals, a balun is typically employed. A balun is an electronic circuit component that converts an unbalanced Radio Frequency (RF) signal at an input port into a balanced RF signal at an output port. In essence a balun is an unbalanced to balanced transformer.
A balun-transformer can be implemented using a number of prior art 180° hybrid structures. A low frequency implementation can be achieved with the use of lumped components with constant reactance. The frequency range of application for this type of balun was recently extended into low-gigahertz frequencies.
Coaxial-line balun transformers have good power handling, but limited bandwidth. These devices are relatively large. As the frequency of application increases, it becomes more difficult to connect the quarter-wave sections in the coaxial-line balun circuit without introducing significant discontinuities that degrade the balun performance. The bandwidth of the best coaxial-line baluns was extended into much lower frequencies by introducing ferrite cores mounted along the outer conductor of a coaxial line. The ferrite cores present a high impedance for the common mode currents along the outer conductors of the balun sections, which corresponds to a good input to output isolation at much lower frequencies.
Due to the growing demand for ultra-broadband balanced circuits and systems in the optical communications and test and measurement industries, there is a growing demand for very broadband 180° hybrid structures that would cover frequencies from well below 1 GHz up to 40 GHz. It would be desirable to provide a single 180° hybrid structure that could operate over this entire bandwidth.